The same happened to my archer several times. From what I can tell, it happened only after being resurrected midfight. I'm quite sure it only happened with Heartseeker equipped, but I can't say I died lots on her after replacing that bow, so it might not be, and probably isn't, weapon specific.
The symptoms, for what I saw, as follows:
-Bugged character graphics (the character may freeze in T shape, with either only armor showing, or whole character visible. Doesn't always happen)
-Bow is equipped in character inventory, and re-equipping seldom helps.
-Marksman special skills with multiple arrows (Arrow Spray, Barrage) only shoot the first arrow.
-Normal "shots" (which most of time use a kicking animation, if the character isn't completely frozen) feel like they do lower damage than they should. Can't say this for sure, tho.
-Most of the time the bug fixes itself as soon as the battle is over, once I had to save/reload after the battle to fix it.
I'm afraid I might not have a save from before the fights where that happened, as I just completed that playthrough today, and they happened a while back, but I'll see if I can re-create it when I have a moment.
Hope this helps.